This invention relates generally to a thermocouple assembly for measuring surface temperatures of tubes and the like, its method of fabrication, and its installation.
In many instances, it is desirable to measure the temperature of a material within a closed tube, such as a tube of a heat exchanger. Accurate temperature measurements have been difficult to obtain because it often is impractical to insert a measuring device through the tube wall for direct measurement of the material temperature. It therefore has been necessary to rely on sensing the temperature of the exterior surface of the tube wall which is indicative of the temperature of the material within the tube. For this purpose, tube skin or surface thermocouple assemblies have been utilized on the skin or exterior surface of the tube to sense the surface temperature of the tube wall.
Many factors, however, make it difficult to obtain accurate temperature measurements of the tubes' exterior surface with such assemblies. The sensing ends of the thermocouple assemblies must be mounted on the exterior surfaces of the tubes where they are subjected to severe environmental conditions, such as high temperature gases and flames. These gases and flames may be of significantly higher temperature than that of the tubes at the points of measurement, and the responsiveness of such thermocouple assemblies may be adversely affected by such higher temperatures.
The effects of the external environmental conditions on thermocouple measurements may be reduced by bringing the measuring junction of the assembly into as intimate contact as possible with the surface of the tube whose temperature is to be sensed and in some way protect or shield the measuring junction from the high temperature gases and flames. Heretofore, this has been done by welding the thermocouple assemblies directly to the surface of the tube. However, known thermocouple assemblies have been costly and difficult to manufacture and/or difficult to weld to the tube wall.
One such type of thermocouple assembly which is welded directly to the tube whose surface temperature is to be sensed is illustrated in U.S. Pat. No. 4,043,200. Such assembly shown requires a difficult to perform manufacturing procedure including, among other things, precise bending of the thermocouple wires of a sheathed thermocouple cable for insertion in a ceramic insulating plug and attachment of a large mass of filler metal to the tip of the thermocouple cable and subsequent machining of the same to provide a chisel-shaped section of metal which is used for welding the assembly to the tube. Another known type of thermocouple assembly is illustrated in U.S. Pat. No. 3,143,439 which also requires a difficult to perform manufacturing procedure including, among other things, attachment of a relatively large, shaped pad to a specially formed end of a sheathed thermocouple cable to permit welding of the same directly to the surface of the tube wall.
In contrast to the above-noted thermocouple assemblies which are welded directly to the tube, oftentimes it is desirable to provide a thermocouple assembly which provides for easy removal and replacement of the same on the surface of the tube. Where the thermocouple assembly is welded directly to the tube wall, removal and replacement of the assembly is difficult, and may result in damage to the tube wall and/or the thermocouple assembly itself.
Thermocouple assemblies which may be easily removed and replaced have also been utilized in the past, but, as in the case where the thermocouple assembly is welded directly to the tube wall, such assemblies have been costly and difficult to manufacture, and in many cases do not ensure shielded intimate contact between the measuring or hot junction of the assembly and the surface of the tube whose temperature is to be sensed. One such type of thermocouple assembly is shown in U.S. Pat. No. 2,607,808 which employs a button-type thermocouple element in a sleeve in a generally U-shaped mounting clamp adapted to be removably secured to the tube. Although such device is readily and easily removed, no provision is made to shield adequately the hot junction from its surrounding environment and hot gases may flow beneath the clamp into contact with the thermocouple element thus leading to inaccurate measurement of the temperature of the tube. Another type of thermocouple assembly is illustrated in U.S. Pat. No. 3,907,606 which employs a mounting block welded to the tube surface. The mounting block is provided with a hole into which is inserted the tip of a sheathed thermocouple cable, and which is then peened tight in the hole to secure the tip. By providing a plurality of such holes, the tip of the thermocouple cable can be replaced without the necessity of welding a new block on the tube. However, a hole must be provided in the block before mounting the same for each foreseeable replacement of the thermocouple.